By looking at this nature guide (Guía de Campo de los Líquenes, Musgos y Hepáticas, by Volkmar Wirth and Ruprecht Dull), I think Acarospora hilaris is the best bet for this species since Pleopsidium flavum is a lichen found in high grounds in Iberia and Acarospora hilaris is a mediterranean species and I found it in a mediterranean environment at low altitude.

The Candelariella medians is really neat (I wish ours were so pretty!), but even that species is dispersed-areolate and not lobate around the margin like this thing. I’d go with Pleopsidium flavum until proven otherwise. (It does occur in Europe according to Nash et al. in Sonoran Flora.)